Tailoring the Properties of Surface-Immobilized Azobenzenes by

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Tailoring the Properties of Surface-Immobilized Azobenzenes by Monolayer Dilution and Surface Curvature Thomas Moldt,† Daniel Brete,† Daniel Przyrembel,† Sanjib Das,‡ Joel R. Goldman,‡ Pintu K. Kundu,‡ Cornelius Gahl,*,† Rafal Klajn,*,‡ and Martin Weinelt*,† †

Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany Department of Organic Chemistry, Weizmann Institute of Science, 76100 Rehovot, Israel

‡

S Supporting Information *

ABSTRACT: Photoswitching in densely packed azobenzene self-assembled monolayers (SAMs) is strongly affected by steric constraints and excitonic coupling between neighboring chromophores. Therefore, control of the chromophore density is essential for enhancing and manipulating the photoisomerization yield. We systematically compare two methods to achieve this goal: First, we assemble monocomponent azobenzene− alkanethiolate SAMs on gold nanoparticles of varying size. Second, we form mixed SAMs of azobenzene−alkanethiolates and “dummy” alkanethiolates on planar substrates. Both methods lead to a gradual decrease of the chromophore density and enable efficient photoswitching with low-power light sources. X-ray spectroscopy reveals that coadsorption from solution yields mixtures with tunable composition. The orientation of the chromophores with respect to the surface normal changes from a tilted to an upright position with increasing azobenzene density. For both systems, optical spectroscopy reveals a pronounced excitonic shift that increases with the chromophore density. In spite of exciting the optical transition of the monomer, the main spectral change in mixed SAMs occurs in the excitonic band. In addition, the photoisomerization yield decreases only slightly by increasing the azobenzene−alkanethiolate density, and we observed photoswitching even with minor dilutions. Unlike in solution, azobenzene in the planar SAM can be switched back almost completely by optical excitation from the cis to the original trans state within a short time scale. These observations indicate cooperativity in the photoswitching process of mixed SAMs.

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INTRODUCTION Self-assembled monolayers (SAMs) are prime candidates for the modification of surface properties such as polarity, chemical reactivity, or charge transfer characteristics at interfaces.1−6 Integration of molecular switches into SAMs is an important issue since it opens the possibility to reversibly change these properties by external stimuli, e.g., light.7−9 Azobenzene represents the most commonly used and investigated molecular switch.10−13 However, directly adsorbed on a metal surface, it exhibits strong substrate-induced quenching of the photoisomerization yield.14,15 Therefore, effective decoupling of the switch from the substrate is required.16 A promising approach is the use of alkyl chains as linkers between the chromophore and the surface,17−19 a strategy we also pursue in this work. Besides the vertical decoupling of the photoswitch from the substrate, one has to account for lateral intermolecular interactions within the SAM. The trans−cis isomerization of azobenzene involves large geometrical changes. In addition, excitonic coupling among the azobenzene molecules in the SAM modifies the optical properties of the ensemble.20 As a consequence, steric hindrance and excitonic band formation are expected to strongly influence the photoisomerization yield.19,21,22 Both © 2014 American Chemical Society

effects can be analyzed and manipulated by tuning the density of the chromophores. For this purpose we employed two strategies: First, we studied SAMs of azobenzene−alkanethiolates on the curved surface of gold nanoparticles (NPs). Placing chromophores on curved surfaces can decrease the chromophore density and thereby enhance the photoisomerization efficiency.13,23,24 As illustrated in Figure 1a, the average chromophore distance increases with decreasing NP size. Consequently, we examined 11-(4-(phenyldiazenyl)phenoxy)undecane-1-thiol25−27 (Az11, structural formula shown in Figure 1b) bound to gold nanoparticles of different sizes. Irrespective of the NP size, we observed pronounced photoswitching, with the changes in optical spectra approximately as large as those for free molecules in solution. Second, on a planar gold substrate, simple (unfunctionalized) alkanethiolate ligands were incorporated into the azobenzene SAM as lateral spacers to decrease the average density of the chromophores. The static dilution of chromophores in planar SAMs has been studied using an asymmetrical disulfide Received: November 4, 2014 Revised: December 18, 2014 Published: December 29, 2014 1048

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moiety tilts toward the surface plane, which cannot occur in densely packed homogeneous domains. In curved SAMs on small NPs the formation of aggregates is likely inhibited by intercalation of solvent molecules. The mixed planar SAMs exhibited reversible photoisomerization, in contrast with the single-component Az11 SAM. The cis and trans photostationary states could be reached within minutes, using power densities of a few mW cm−2 that could be readily delivered by LEDs. The cis form was stable for hours in the dark under ambient conditions, and mixed SAMs could be switched for several cycles without appreciable fatigue. Despite the fact that we illuminated the diluted SAMs with a photon energy corresponding to the main absorption band of Az11 in solution, we always observed the largest spectral change in its blue-shifted aggregate band. On the basis of these results, we conclude that the photoisomerization of Az11 in mixed SAMs takes place in a stepwise cooperative process.

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Figure 1. (a) Schematic representation of azobenzene−alkanethiolates on a flat gold surface and on the curved surfaces of two differently sized nanoparticles. The density of chromophores is largest for the planar substrate and decreases with decreasing NP diameter. (b) Structural formulas of the compounds used in this work.

EXPERIMENTAL SECTION

All experiments involving azobenzene compounds were carried out under yellow light with a cutoff wavelength of 500 nm, well above the absorption bands relevant for azobenzene photoisomerization. Sample preparation, DR measurements, and illumination experiments were conducted under ambient conditions. Az11 and 11-phenoxyundecane1-thiol (P11, cf. Figure 1b) were synthesized as described in the Supporting Information. 1-Dodecanethiol (98%, C12) was used as obtained from Alfa Aesar. NP Synthesis. Fairly monodisperse gold NPs of various diameters (≈ 2.5−12 nm) were synthesized using a previously described technique.38 Briefly, 2.58 nm NPs were prepared by reducing a toluene solution of HAuCl4 with tetrabutylammonium borohydride in the presence of surfactants (see Supporting Information for detailed procedures). These small NPs were functionalized with thiols or used as seeds for the synthesis of larger particles, up to 12 nm in diameter. As-prepared NPs were stabilized with dodecylamine (DDA) and didodecyldimethylammonium bromide (DDAB)weakly bound ligands that could readily be displaced with thiols in a place-exchange reaction. The ligand exchange did not affect the sizes of the particles. Preparation of Curved SAMs on NPs. The obtained NPs were functionalized with monocomponent monolayers of either Az11 or P11. Transmission electron microscopy (TEM) images of the functionalized NPs are shown in Figure 2. Following precipitation and a thorough washing to remove any excess of unbound thiols, we tested the solubility of Az11-functionalized NPs in a variety of solvents. We chose chloroform as optimal solvent because it stabilizes NPs coated with both trans and cis isomers of Az11; i.e., the UVinduced isomerization is not accompanied by aggregation of NPs.39−41 Unfortunately, even chloroform could not dissolve Az11-functionalized NPs larger than 8 nm, for which no suitable solvent was identified. This poor solubility can be attributed to the densely packed monolayers of π−π stacked azobenzene groups that the solvent molecules could not solvate.a Preparation of Planar SAMs. We prepared SAMs on 300 nm thick gold films on thin sheets of mica that had been annealed after gold deposition (Georg Albert, PVD coating). The polycrystalline surfaces exhibit large Au(111) terraces of a few hundred nanometers in width.42 For SAM preparation, immersion solutions of a 0.1 mM total thiol concentration in methanol were prepared by mixing and diluting the appropriate amount of stock solution of each component. After immersion for 20 h, the samples were copiously rinsed with methanol and blown dry with argon. XPS and DR measurements were carried out on twin samples cut from the same piece of gold/mica substrate and immersed back-to-back into the same solution. The concentration of the thiol solutions can be assumed to remain constant during SAM formation because a 150-fold excess of thiol was employed.b In order to verify the quality of the SAMs, sulfur 2p XP spectra were recorded for all samples. From samples examined at the synchrotron

consisting of an azobenzene-terminated and an unfunctionalized alkanethiolate counterpart; Tamada and co-workers showed that upon adsorption on planar gold the S−S bond breaks and leaves behind two thiolates, corresponding to a 50% dilution of azobenzene.28 A variable dilution of the azobenzene chromophores is possible by coadsorption from a solution of two thiols.13,29 The main problem of using this approach is that one must ensure proper mixing of the two structurally different constituents on the surface. Sometimes the different thiolates segregate and form islands,30,31 or one component is even fully displaced from the surface.32 An earlier publication showed indications of photoswitching in a mixed SAM where the azobenzene chromophores were strongly diluted.33 Recently, azobenzene SAMs of different dilutions have been prepared by coadsorption of two thiols: Photoisomerization in the resulting mixed SAMs was observed using surface plasmon resonance spectroscopy,29 photoelectrochemical measurements,29,34 vibrational sum-frequency generation,35 scanning tunneling microscopy,36 and surface-enhanced Raman spectroscopy.37 In this work we applied X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and UV/vis differential reflectance (DR) spectroscopy in order to examine bicomponent SAMs of Az11 and 1-dodecanethiol (C12) on planar gold substrates, prepared by coadsorption from solution. XPS allowed us to determine the relative Az11 coverage in the bicomponent SAMs. The coverage could be tuned between 0 and 100% by adjusting the mole fractions in solution, despite an observed preferential adsorption of Az11. UV/vis spectroscopy of both curved and planar SAMs revealed pronounced excitonic shifts of the optical absorption bands compared with the free molecule. This effect increases with increasing chromophore density, which indicates the tunability of the coupling between the chromophores and thus their optical properties. On planar SAMs we found hints of small Az11 aggregates at low azobenzene densities already. However, segregation of the mixed planar SAMs into large C12 and Az11 domains did not occur. This was corroborated by determining the molecular orientation using NEXAFS spectroscopy. Upon dilution of Az11 with C12 the azobenzene 1049

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on the sample: 365 (9) nm, 3.40 (0.08) eV, 7 mW cm−2 and 460 (26) nm, 2.70 (0.15) eV, 12 mW cm−2. Chloroformic solutions of NPs were illuminated with a mercury discharge lamp (365 nm, 1 mW cm−2). UV/vis spectra of nanoparticle solutions were recorded on a Shimadzu UV-2700 spectrophotometer using standard quartz cells with an absorption length of 10 mm. Spectra of P11-coated nanoparticles were subtracted from those of Az11-coated nanoparticles (see details below).

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RESULTS AND DISCUSSION Curved SAMs on Nanoparticles. Optical Properties. The UV/vis absorbance spectra of Az11-decorated NPs (Az11-NPs) and free Az11 are shown in Figure 3a in the range of the dominating azobenzene S2 (π−π*) absorption band at 3.5 eV (354 nm). A comparable contribution to the absorbance of NPs originates from the localized surface plasmon resonance (LSPR) at a photon energy of 2.4 eV (520 nm),46,47 which increases with increasing NP diameter and masks the weak S1 absorption band of Az11 at 2.7 eV (460 nm). To separate the optical response of the surface-bound chromophores from that of the gold NPs, we functionalized the same batches of NPs with a nonabsorbing ligand. For this purpose, the phenoxyterminated alkanethiol (P11) was chosen because it endows the particles with excellent solubility in chloroform and is structurally similar to Az11 (cf. Figure 1b) while not absorbing

Figure 2. Transmission electron microscopy (TEM) images of Az11functionalized gold nanoparticles of different diameters. All images were taken at the same magnification; the scale bars correspond to 10 nm. The errors given are the standard deviations in the NP diameter distribution. source (for the spectra see Supporting Information), we could conclude that the main contribution to the spectra originated from the gold-bound thiols, as expected. The peak area commonly attributed to atomic sulfur was well below 10% of the total peak area, whereas there was no indication for unbound thiol. For the samples examined with the lab-based XPS setup, contributions of atomic sulfur and unbound thiol were below the S 2p detection limit of 10% of the total peak area. X-ray Spectroscopy. XPS and Auger yield NEXAFS spectroscopy were carried out at the beamline UE56-2_PGM-2 of the synchrotron facility BESSY II (Helmholtz-Zentrum Berlin), using an ultrahighvacuum (UHV) apparatus described in an earlier work.19 Additionally, lab-based XPS measurements were performed at room temperature in a UHV setup consisting of a mu-metal chamber equipped with a monochromatized X-ray source (VG Scienta MX650, Al KαI, hν = 1486.7 eV) and a high-resolution electron analyzer (VG Scienta SES200). The background pressure during all measurements was below 2 × 10−10 mbar. The radiation was focused on a sample area of 3 × 9 mm2. The samples were oriented toward the detector for a mean takeoff angle of 65° with respect to the surface normal. The total energy resolution was better than 400 meV. All XP spectra were referenced to the substrate Au 4f7/2 peak at a binding energy of 83.96 eV.44 To evaluate peak positions and peak areas, the signals were fitted with Voigt line profiles and Shirley45 backgrounds. To compensate for X-ray intensity losses due to the aging of the laboratory X-ray source, the spectral intensities were corrected according to regularly measured reference samples. UV/Vis Measurements. Absorption spectra of Az11 in methanol were recorded at a concentration of approximately 0.03 mM in cells with an absorption length of 10 mm. Differential reflectance (DR) spectra of SAMs were measured using a PerkinElmer Lambda 850 spectrometer. For this purpose a reflection unit was designed, which allows measurements using s- and p-polarized light to get insight into the average molecular orientations. The SAM-induced change in reflectivity was determined from the difference between the reflectivity of the gold substrate before and after SAM preparation (see Supporting Information). Illumination experiments on Az11 in solution and on SAMs with normal incidence were carried out using two LED sources with the following central wavelengths and photon energies (full width at half-maximum in parentheses) and intensities

Figure 3. UV/vis spectra of gold nanoparticles (NPs) with different diameters, in comparison with free Az11 in chloroform (dashed): (a) Az11-functionalized NPs, normalized to the S2 band height; offsets were added. The peak at ≈2.4 eV (520 nm) originates from a localized surface plasmon resonance (LSPR of gold NPs). (b) P11-functionalized NPs, normalized to the LSPR band heights in (a). (c) Photoswitching experiment on Az11-functionalized NPs, pristine (black) and the photostationary state (PSS) at 365 nm (gray). The signal originating from the surface plasmon resonance has been subtracted, and the resulting difference spectra were normalized to the S2 peak height. 1050

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43% dilution of a SAM on a flat gold substrate. Therefore, we systematically tested mixing Az11 and C12 to create SAMs with a chromophore density similar to the density on NPs and to enable efficient photoswitching. Structure and Optical Properties of Planar Bicomponent SAMs on Au(111). Composition of Mixed SAMs. Bicomponent SAMs on Au(111)/mica were made from mixed solutions of Az11 and C12, with varying mole fractions χsol of Az11. We analyzed the miscibility and component ratio of the two thiol species in SAMs by XPS of the N 1s core level. As depicted in Figure 5, the Az11 diazo bridge gives rise to a single photoemission peak. Its intensity decreases upon dilution of Az11, whereas its energetic position shifts continuously toward higher binding energies (for a plot of the peak positions, see Supporting Information). The change in binding energy results from electrostatic interaction of the molecules with their environment. The continuous shift indicates largely statistical mixing of the two thiol species: In the case of two-dimensional island growth, we could not obtain a continuous shift of the XPS binding energy when we changed the Az11 coverage from 0 to 100%, as substantiated in a previous study on mixed SAMs of azobenzene-alkanethiols with fluoromethyl and cyano end groups.50 The N 1s XPS peak area represents the relative Az11 coverage Θ of a mixed SAM, when normalized to the peak area of a single-component Az11 SAM. Nitrogen atoms are only

light above ≈280 nm. In the spectra of these P11-functionalized NPs (Figure 3b) the optical response originates from the metallic cores of the particles, which is altered by the SAM acting as a dielectric layer. We subtracted this “background” from the spectra of Az11-decorated NPs for varying curvatures (the solid lines in Figure 3c). To determine the number of adsorbed molecules, N = 4πR2/ F0, on a NP of radius R, we assumed a footprint of F0 = 0.217 nm2 of alkanethiolates25 on gold(111).c Then we calculated the chromophore density, ρ = N/4π(R + d)2, in a sphere of radius R + d, where d is the distance between the chromophore and the NP surface. For d we chose the distance between the Au atom of the substrate and the “lower” N atom in the diazo bridge (Figure 1b) of 2.25 nm, assuming that the molecules have a stretched upright geometry. In Figure 4 the S2 band position maxima for pristine NPs (obtained from Figure 3c) are plotted versus the chromophore density, ρ. The S2 band shifts to higher energies with increasing chromophore density. This hypsochromic (blue) shift is attributed to the formation of H-aggregates, as discussed later on in the comparison of excitonic shifts in planar and curved SAMs.

Figure 4. S2 band position on NPs and in mixed SAMs versus chromophore density, ρ. The S2 band position of Az11 in solution is added for comparison. All data points correspond to absorption maxima directly read from the spectra. The solid line represents a fit to the SAM data points, the dashed line has the same slope, and an offset was added to compare with the NPs.

Photoisomerization. We examined the trans−cis photoisomerization of Az11 on NPs and compared it to the photoisomerization of free Az11 in solution. The bottom part of Figure 3c shows the absorbance spectrum of trans-Az11 in solution and a spectrum after illumination with UV light (365 nm). The UV light triggers the trans-to-cis isomerization. This leads to a weakening and a hypsochromic shift of the S2 band, whereas the S1 transition appears slightly stronger. Because the S2 bands of trans and cis isomers partially overlap, only a photostationary state (PSS) with residual 10−15% of the trans isomer is achieved.18 Also on the NPs we observed pronounced photoisomerization of Az11. The photoinduced relative change in the S2 band intensity is very similar to that of free Az11, indicating a comparable photoswitching efficacy. For NPs of smaller diameters, the relative change of the S2 band increases only slightly. Thus, it can be concluded that a lower chromophore density results in a little more effective photoswitching. The maximum density studied on the NPs corresponds to around

Figure 5. Series of N 1s XP spectra of mixed SAMs for different mole fractions χsol(Az11) in the adsorption solution. The fits (solid lines) were obtained as described in the Experimental Section. 1051

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Langmuir present in Az11 and the attenuation of the signal due to scattering at the outer phenyl ring should be weak. Therefore, changes in the molecular orientation are expected to have a negligible influence on the recorded N 1s XPS intensity. The coverage Θ is not identical to the mole fraction of Az11 in the SAM because the footprint of alkanethiolates is about 10% smaller than the footprint of azobenzene-functionalized thiolates.d Therefore, the total number of thiolates in the SAM increases slightly while diluting Az11 with C12. The relation between the mole fraction χsol of Az11 in the adsorption solution and the Az11 coverage Θ in the SAM is plotted in Figure 6. For mole fractions χsol ≤ 20%, the mixture

Figure 7. Transition dipole moments (TDMs) in Az11. (a) The TDM of the π* transition probed in NEXAFS is oriented perpendicular to the aromatic plane. α denotes the angle between the surface normal n and the normal of the aromatic plane. ϑ is the angle between the surface normal and the axis through the N−C bond at the “upper” phenyl ring. (b) The optical TDMs μ(S2) and μ(S3) lie in the aromatic plane.19

approach has already been successfully applied to SAMs of biphenyl-based thiols on gold(111).53 In aromatic systems, the TDMs of excitations into π* orbitals are perpendicular to the ring plane.52 This allows us to determine the mean tilt angle α between the normal of the ring plane and the surface normal, defined in Figure 7a. Figure 8 shows N 1s NEXAFS spectra for SAMs of varying Az11 coverage Θ. The π* (LUMO) resonance at a photon energy of 398.4 eV was used to determine the molecular orientation. For the 100% Az11 SAM the Auger yield is stronger for s-polarized than for p-polarized light; however, this polarization contrast is inverted for smaller coverages Θ, which indicates a significant change in the orientation of the chromophores. Evaluating the polarization contrast of the peak areas yields the tilt angles α compiled in Table 1. The

Figure 6. Az11 coverage Θ in mixed planar SAMs plotted versus the mole fraction χsol(Az11) in solution. The coverage Θ was determined from the peak area of the N 1s XP spectrum. The dashed line represents ideal mixing, and data points above the line indicate preferential adsorption of Az11.

on the surface resembled that in solution (dashed line); i.e., we found a nearly linear relation. At higher χsol, however, the Az11 coverage in the SAM exceeded the mole fraction in solution. This preferential adsorption of Az11 most likely results from the different interactions between the molecules in the SAM. The van der Waals interaction of C12 with neighboring C12 or Az11 should be very similar because the length of the aliphatic chains is nearly the same. In contrast, two neighboring Az11 molecules should interact more strongly due to additional π−π interactions of the chromophores. This leads to preferential adsorption of Az11 with increasing mole fraction of Az11 in solution, however, without substantial segregation and formation of large homogeneous domains. These processes may be suppressed because of the smaller footprint of C12 which leads to a gain in the total adsorption energy due to an increase of the total thiol coverage when C12 mixes with Az11. An immersion time of 20 h was sufficient to reach the equilibrium composition in the SAM.e Orientation of the Azobenzene Moieties. In order to study the chromophore orientation for different Az11 coverages Θ, we applied near-edge X-ray absorption fine structure (NEXAFS) spectroscopy by recording the Auger yield. More specifically, we analyzed excitations from the C 1s and N 1s core levels into the unoccupied molecular orbitals. We used a method developed by Stö hr et al.52 to determine the orientation of the transition dipole moment (TDM) with respect to the surface normal from the X-ray polarization contrast (for details see Supporting Information). This

Figure 8. N 1s NEXAFS of SAMs with varying Az11 coverage Θ, for sand p-polarized light, and light polarized along the magic angle.f The peak at 398.4 eV originates from the excitation into the π* (LUMO) orbital, whereas the peaks in the [400, 403] eV range are assigned to excitations into higher unoccupied π* orbitals. The broad feature around 407 eV is a σ* resonance. Assignments are in accordance with earlier work for a similar molecule.19 1052

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Langmuir Table 1. Average Orientations α (cf. Figure 7a) in SAMs of Varying Az11 Coverage Θ, Determined from N 1s and C 1s NEXAFS Spectraa α/deg Θ(Az11)/%

N 1s

C 1s

100 ≈80 ≈15

73 ± 5 59 ± 5 45 ± 5

71 ± 5 55 ± 5 42 ± 5

a

The error of 5 includes systematic contributions and sample-tosample variations.

same method was applied to the C 1s π* resonances (see Supporting Information). In NEXAFS spectroscopy we average over a macroscopic area of the sample. Thus, this method provides only the mean tilt angle α of the aromatic plane, without the information on how strongly every single molecule deviates from the average orientation. In contrast to our earlier works on SAMs of azobenzene functionalized with specific end groups at the 4′-position,19,50 we cannot determine the angle ϑ of Az11 (see Figure 7a), which gives approximately the orientation of the long molecular axis with respect to the surface normal. But since α for the 100% Az11 SAM is in excellent agreement with the aforementioned results, we expect an analogous angle ϑ of about 30° for the azobenzene moiety.g We can conclude that for the 100% Az11 SAM the plane of the phenyl rings is oriented predominantly perpendicular to the surface, whereas upon dilution, the angle α decreases significantly. This must coincide with an increase in ϑ and thus with a “bending down” of the azobenzene units upon dilution. Such a change in the average orientation in the mixed SAM would not occur in the case of the formation of extended densely packed domains, which corroborates the largely statistical mixing of Az11 and C12 molecules, as deduced from the continuous N 1s peak shift in XPS. The NEXAFS contrast and the tilt angle, α, change significantly even for a small dilution to 80%. Our NEXAFS data are compatible with the range of tilt angles ϑ of 45°−60° determined in a complementary work35 for mixed SAMs of C12 and trans-p-cyanoazobenzenethiols.h Optical Properties of Planar SAMs. Because the gold substrate is nontransparent, we used differential reflectance (DR) spectroscopy in order to examine the optical properties of planar SAMs. More specifically, we measured the change in reflectivity caused by the SAM on the gold surface (for details of the method see Supporting Information). The DR spectra were recorded at an angle of incidence of 45° with respect to the surface normal, using linearly polarized light. Figure 9 shows DR spectra of mixed and single-component SAMs measured with p- and s-polarized light. The electric field vector of s-polarized light is parallel to the surface, whereas p-polarized light at 45° has components parallel and perpendicular to the surface (see schematic drawings in the figure). The general shape of the DR spectra is given by the change in reflectivity due to the SAM acting as a dielectric layer.19 This effect is best discussed for the C12 SAM, which contains no chromophores (Θ = 0%), and the spectrum lacks any absorption bands. We observed a drop in the DR signal at 2.6 eV, a step at 3.6 eV, and an increase for photon energies higher than ≈4.5 eV. In the middle of the figure, an absorbance spectrum of trans-Az11 in solution is plotted. The observed absorption bands are typical18,54,55 for azobenzenes. Like in the SAMs on NPs the dominating transition is the S2(π−π*) excitation; the S1(n−π*)

Figure 9. Series of p- and s-polarized differential reflectance (DR) spectra of SAMs for different Az11 coverages Θ (angle of incidence 45°, offsets were added). Schematic drawings indicate the orientation of the electric field vector of the incident light. An absorbance spectrum of trans-Az11 in methanol (middle, dashed) is plotted for comparison. The inset shows the upscaled S1 absorption band in solution.

excitation is only weakly observed.i The S3 absorption band is due to higher π−π* excitations. As expected, the p-polarized DR signal of the S2 band (top) increases with increasing Az11 coverage Θ. Additionally, the absorption maximum shifts toward higher photon energies. This hypsochromic shift amounts to 0.64 ± 0.01 eV for a 100% Az11 SAM compared with the band position in solution. The S3 absorption band also increases in intensity with increasing Θ. In contrast to the S2 band, the center of the S3 band shifts toward lower energies (bathochromic shift) and a weak fine structure of three peaks emerges. The observed shifts in the S2 and S3 bands can be understood using the theory of excitonic coupling.20 The S2 transition dipole moment (TDM) of an isolated trans-Az11 molecule lies in the plane of the chromophore, almost parallel to the axis that connects the C4 and the C4′ atoms (see Figure 7b). We have shown that in a 100% Az11 SAM the chromophores, and thus the S2 TDMs, stand predominantly upright. In a 2D arrangement, they form an H-aggregate, leading to a hypsochromic shift of the respective band.20 The formation 1053

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Langmuir of H-aggregates has been observed for many azobenzenefunctionalized SAMs.19,56,57 The excitation into the S2 band is strongest when the electric field vector of the incident light is oriented parallel to the TDM; thus, the S2 transition is much more pronounced in p- than in s-polarized spectra. The S3 band in Az11 SAMs exhibits a bathochromic shift with respect to that in solution, in contrast to the S2 band. This shift can be explained by the formation of a J-aggregate, which is composed of head-to-tail aligned TDMs.58 The S3 TDMs are oriented roughly parallel to the NN bond (see Figure 7b). The S3 band is only slightly more intense in p- than in spolarized DR spectra; therefore, the NN bond has an intermediate orientation between perpendicular and parallel to the surface. In J-aggregates vibrational excitations are suppressed,59 revealing the absorption fine structure visible for the 100% SAM. In addition to the blue-shifted part of the S2 band, the ppolarized DR spectra show a signal in the energy range of the S2 band of Az11 in solution (≈3.6 eV). We attribute this mainly to the absorption by molecules located at “defects”, e.g., in the vicinity of C12 or on gold step edges. Another contribution stems from the dielectric background. Comparison of Excitonic Shifts in Planar and Curved SAMs. Both planar and curved SAMs show density-dependent excitonic shifts of the S2 band. Figure 4 shows a plot of the observed S2 band positions versus the chromophore density, ρ. In planar SAMs ρ is proportional to the coverage Θ.j For planar SAMs with vanishing ρ, one might expect an S2 band position similar to that of free Az11 in solution. However, a linear fit yields an offset of (0.21 ± 0.02) eV. This value is one order of magnitude larger than typical solvatochromic effects.18 We can conclude that the chromophores in the planar SAM tend to form small aggregates even for very low chromophore densities.k This view is corroborated by the fact that models of azobenzene dimers and oligomers21 predict an excitonic shift of this order of magnitude. A similar trend of the excitonic shift increasing with chromophore density, ρ, can be observed for curved SAMs. In contrast to the planar SAMs, the extrapolation to infinitely small ρ values is in agreement with the S2 band position of Az11 in solution. Thus, for small NPs the formation of aggregates might be inhibited by solvation of the azobenzene units, in contrast with the planar SAMs, where no solvent is present. This is in agreement with the observation that NPs with diameters larger than ≈8 nm are insoluble in all solvents. Photoisomerization in Planar SAMs. Figure 10 shows ppolarized DR spectra of pristine SAMs and spectra after illumination with UV (365 nm) and blue (460 nm) light in comparison with the analogous photoswitching experiment of Az11 in solution. The illumination with UV light triggers the trans-to-cis isomerization. In solution this leads to a weakening and a hypsochromic shift of the S2 band, whereas the S1 transition appears slightly stronger because the respective transition is not symmetry-forbidden in the cis isomer. The S2 bands of the trans and cis isomers partially overlap; therefore, only a photostationary state (PSS) with residual 10−15% of chromophores in the trans form is reached. 18 Upon illumination with blue light, we reach another PSS that is distinct from the initial state (i.e., the PSS contains residual cis isomer). Experimentally, the samples were illuminated until no further change in the spectrum was observed. The following photon doses were sufficient to reach a PSS in solution: 3 × 1017 photons cm−2 for UV light and 9 × 1017 photons cm−2 for

Figure 10. Photoswitching experiment on Az11 SAMs and Az11 in solution; pristine and photostationary states (PSS) after illumination with UV (365 nm) and blue (460 nm) light are shown. Top: ppolarized DR spectra of SAMs for different coverages Θ; vertical offsets were added for clarification. Bottom: absorbance spectra of Az11 in methanol. The illumination wavelengths are indicated by vertical lines.

blue light. These photon doses are equivalent to a few seconds of illumination with the LED lamps. For the 100% Az11 SAM, very small changes upon UV illumination were observed with a total photon dose of 1019 photons cm−2 (several minutes). This is in line with previous work on densely packed azobenzene SAMs.19,22 In contrast with the 100% Az11 SAM, lower-coverage SAMs exhibit a pronounced photoswitching. This can be seen best from the change in the DR signal at the S2 band. For decreasing coverage Θ, i.e., for a higher dilution of Az11, a larger percentage of chromophores can be optically switched. In each case 6 × 1018 photons cm−2 were sufficient to reach the UV PSS. Upon illumination with blue light, we reached a PSS after a photon dose of 8 × 1018 photons cm−2, equivalent to a few minutes of illumination time. We performed several switching cycles on mixed SAMs; no appreciable fatigue could be observed (see Supporting Information). The center wavelength of the UV light source used for this work is 365 nm (3.4 eV)close to the S2 absorption band maximum of the Az11 monomers in solution. However, in all switching SAMs we observed that illumination at this wavelength has a significant effect on the S2 aggregate band (peak at ≈295 nm, 4.2 eV). This difference is very large compared with the full width at half-maximum of the LED source of 80 meV. The response of the aggregate band when exciting the monomer transition suggests a stepwise cooperative switching process of azobenzene stacks within the mixed 1054

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Langmuir SAMs: isomerization of a chromophore at the edge of an aggregate reduces the coupling to its neighbor. Consequently, it alters the latter’s absorption and opens space for switching. This describes a scenario where chromophores in the SAM switch one after another in a domino-like manner. Furthermore, switching the SAM back with blue light leads almost to the pristine spectrum, which is in contrast to the blue PSS in solution, in which a substantial amount of the cis isomer can be observed. We neglected thermal relaxation from the cis into the trans ground state during the experiment because spectra of mixed SAMs in the UV PSS were stable for several hours after turning off the light source. The trans/cis ratio of the blue PSS depends on the absorption cross section σ and on the photoisomerization quantum yield, Φ, of both isomers. The absorption cross section, σ, is a measure for the probability of a chromophore to absorb a photon; Φ yields the probability of isomerization upon absorption. The preferential adsorption of Az11 and the zero-density offset of the S2 band position with respect to that in solution indicate that the trans state is energetically favored in the SAM due to the interaction of a chromophore with its neighbor. This gives rise to a difference in the photoisomerization quantum yield between SAM and solution. Upon illumination with blue light, the ongoing cis-totrans isomerization in the SAM leads to an increasing preference of the trans species. We can conclude that lateral interactions foster the complete back switching of the SAM. This corroborates the proposed cooperativity in the photoisomerization of our SAMs, in agreement with STM studies on the photoisomerization of azobenzene SAMs.60−62

steric hindrance, which promotes further switching. The blue PSS is almost identical to the pristine state. The cis state is stable for at least several hours under ambient conditions, and no appreciable fatigue was observed after several switching cycles. Our results demonstrate that tuning the density and composition of azobenzene SAMs enables effective photoswitching and thus paves the way toward tailoring surface and interface properties in a reversible fashion.

SUMMARY AND CONCLUSIONS Steric hindrance and excitonic coupling in azobenzene− alkanethiolate (Az11) SAMs can be reduced by decreasing the chromophore density. This has been demonstrated using two approaches: first, we bound Az11 to microscopically curved surfaces, i.e., gold nanoparticles; second, we prepared mixed SAMs on planar substrates by coadsorption from a solution of two thiol species. We demonstrated that Az11 and C12 form mixed layers and that the average orientation of the chromophores changes from an almost upright-standing position to a more flat-lying one upon dilution of Az11 with C12 spacer molecules. Increasing the chromophore density in curved singlecomponent Az11 SAMs by changing the NP diameter leads to an increasing hypsochromic shift of the azobenzene S2 absorption band. A similar density dependence is found for the excitonic coupling among the chromophores in mixed planar SAMs. An offset between planar and curved SAMs indicates the formation of small aggregates in mixed SAMs having low Az11 coverage, but there is no sign of further segregation and the formation of large single-component islands at higher coverages. The photoswitching of Az11 bound to nanoparticles is as effective as for Az11 in solution. In contrast, the 100% Az11 SAM on a planar substrate exhibits negligible photoisomerization yield. The dilution of Az11 with C12 is a prerequisite for efficient and reversible photoswitching of the planar SAM, with photostationary states reached after illumination with (6−8) × 1018 photons cm−2. Coverage dependence, spectral response, and full reversibility of the trans-to-cis photoisomerization all indicate cooperativity in a step-by-step switching process. Isomerization from trans to cis of a given chromophore leads to decoupling of the neighboring Az11 molecules and reduces

ACKNOWLEDGMENTS Support by the Deutsche Forschungsgemeinschaft through Sfb 658Elementary Processes in Molecular Switches at Surfaces, the Helmholtz Virtual InstituteDynamic Pathways in Multidimensional Landscapes, the Israel Science Foundation, and the Minerva Foundation are gratefully acknowledged.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of Az11 and P11; preparation of planar and curved SAMs; additional UV/vis, XP, and NEXAFS spectra; method used to determine the molecule orientation from NEXAFS spectra; setup used for UV/vis DR spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (C.G.). *E-mail [email protected] (R.K.). *E-mail [email protected] (M.W.). Present Address

S.D.: Dept. of Colloids and Materials Chemistry, CSIR-Institute of Minerals and Materials Technology, Odisha, India. Notes

The authors declare no competing financial interest.

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ADDITIONAL NOTES This view is corroborated by the observation that NPs coated with mixed monolayers of Az11 and small amounts of P11 were highly soluble in a wide range of solvents even at large particle sizes (e.g., 12 nm). b We used 1 mL of 0.1 mM thiol solution per cm2 of sample, and assumed a surface area of 0.242 nm2 per Az11 molecule.43 c On NPs with about 2 nm diameter, footprints of about 0.15− 0.17 nm2 have been reported.48,49 For larger NPs the footprint should converge with that of planar gold. Using the value for planar gold leads to a small error for small NPs, without affecting the general trend. d Simple alkanethiolates on Au(111) form a (√3 × √3)R30° structure,25 which leads to a footprint of 0.217 nm2, whereas for SAMs of Az11 and also the equivalent compound with a shorter alkyl chain (6-4-[phenyldiazenyl]phenoxyhexane-1-thiol) a value of about 0.242 nm2 has been determined.43,51 e Samples that had been immersed for just 30 min or up to 8 days showed no changes in Az11 coverage compared with samples immersed for 20 h. f The magic angle is the polarization angle for which the X-ray absorption is independent of the molecular orientation. g The relation of ϑ to α depends on the twist angle γ of the chromophore.19 h Here, a longer linker of mercaptopentadecanoic ester was used for the azobenzene compound. a

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The n−π* excitation is symmetry-forbidden in transazobenzene, but it is weakly visible due to molecular vibrations.54 In Az11 it is more intense because of the asymmetric substitution. j The chromophore density was calculated using a footprint of 0.242 nm2 for Az11.43 k The formation of larger aggregates would contradict the previously shown XPS and NEXAFS results.

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